The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to steady state free precession (SSFP) methods for acquiring MRI data and suppressing fat signal in reconstructed images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most MRI scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (“TR”) and result in complete scans that can be conducted in seconds rather than minutes. Whereas the more conventional pulse sequences have repetition times, TR, which are much greater than the spin-spin relaxation constant, T2, so that the transverse magnetization has time to relax between the phase coherent excitation pulses in successive sequences, the fast pulse sequences have a repetition time, TR, which is less than T2 and which drives the transverse magnetization into a steady-state of equilibrium. Such techniques are referred to as steady-state free precession (“SSFP”) techniques.
With the recent introduction of high performance gradient systems on commercially available MRI systems, these SSFP imaging pulse sequences have received more attention. Not only do they significantly shorten scan time, but they also have relatively high signal-to-noise ratio (“SNR”) while providing T2-like contrast based on the T2/T1 ratio of tissues.
Two major problems are associated with the SSFP acquisition method. First, the images produced have undesirably bright lipid signals due to the high T2/T1 ratio of fat spins. The bright signal complicates clinical interpretation and obscures nearby tissues of greater clinical significance. Second, when using SSFP pulse sequences signal dropout and banding artifacts can appear in regions of B0 field inhomogeneity. To reduce banding artifacts and maximize signal-to-noise ratio (“SNR”) efficiency, an extremely short repetition time (“TR”) is usually desired.
Two methods to suppress fat in SSFP images are described in U.S. Pat. No. 6,307,368. In the Fluctuating Equilibrium MR (“FEMR”) method, RF phase cycling creates transverse magnetization that fluctuates between water and fat signal on alternating pulse sequences. The second method, Linear Combination SSFP (“LCSSFP”), acquires two image datasets with SSFP pulse sequences using different RF phase cycles and then linearly combines the datasets during the image reconstruction. With this approach, image data sets can be combined differently to create both fat and water images without a loss in SNR efficiency.
To operate properly the FEMR and LCSSFP fat suppression methods require the use of a SSFP pulse sequence having a very short repetition period (“TR”). Both FEMR and LCSSFP work best when a 180 degree phase shift occurs between fat and water spins during each TR interval. The ideal repetition time for perfect fat water separation at a B0 field strength of 1.5 Tesla (“T”), therefore, is approximately 2.2 milliseconds (“ms”). However, obtaining such a short TR is difficult without sacrificing readout resolution, which limits the applicability of the method.
By linearly combining the two echoes as described, for example, in U.S. Pat. No. 7,148,685, either fat or water suppression can be achieved. However, the unwanted species is not consistently suppressed. Using this method, either fat or water suppression can be achieved, as shown in FIG. 11A. However, the unwanted species is not suppressed as well as in conventional LC-SSFP, as shown in FIG. 11B. Despite this, the desired passband is still much wider than conventional LC-SSFP, as shown in FIG. 11A. Furthermore, while the phases of the suppressed band can be discriminated, as shown in FIG. 11C, their variation is not uniform across the suppression band. As a result, the phase information contained in the linearly combined image cannot be accurately used to suppress fat.
It would therefore be desirable to provide a method for water-fat separation having a desired passband wider than in conventional LC-SSFP methods, but with greater suppression of unwanted MR signals than previous methods, such as those described in U.S. Pat. No. 7,148,685.